Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts

Abstract Suppressing the amount of carbon dioxide in the atmosphere is an essential measure toward addressing global warming. Specifically, the photocatalytic CO2 reduction reaction (CRR) is an effective strategy because it affords the conversion of CO2 into useful carbon feedstocks by using sunlight and water. However, the practical application of photocatalyst‐promoting CRR (CRR photocatalysts) requires significant improvement of their conversion efficiency. Accordingly, extensive research is being conducted toward improving semiconductor photocatalysts, as well as cocatalysts that are loaded as active sites on the photocatalysts. In this review, we summarize recent research and development trends in the improvement of cocatalysts, which have a significant impact on the catalytic activity and selectivity of photocatalytic CRR. We expect that the advanced knowledge provided on the improvement of cocatalysts for CRR in this review will serve as a general guideline to accelerate the development of highly efficient CRR photocatalysts.


Photocatalytic CO 2 reduction
Since the Industrial Revolution in the late 18th century, humankind has consumed large quantities of fossil fuels (coal, oil, and natural gas). The use of fossil fuels generates nitrides and sulfides, which are harmful to the ecosystem, cause air pollution, and increase greenhouse gas emissions (e. g., carbon dioxide), which ultimately lead to climate change. Therefore, reducing the use of fossil fuels and thereby suppressing CO 2 emissions are pressing issues that need to be addressed to prevent global warming. In addition, given the current situation in which it is not possible to promptly and completely stop the use of fossil fuels and shift to a society that uses alternative greener fuels (e. g., hydrogen, ammonia) as its energy source, it is essential to establish means to reduce CO 2 concentration in the atmosphere in parallel with reducing the use of fossil resources. As a strategy to reduce atmospheric CO 2 concentration, a technology to capture and bury emitted CO 2 underground has been established and is currently being applied. Alternatively, as a more efficient strategy, the emitted CO 2 could be recycled (carbon cycle) through its conversion into carbon monoxide (CO) and organic compounds (synthesis gases and hydrocarbon compounds such as raw materials for chemicals, agrochemicals, and pharmaceuticals; Figure 1).
To reduce CO 2 to useful compounds (CO 2 reduction reaction (CRR)), photochemical reduction using sunlight and water (H 2 O) and electrochemical reduction using H 2 O are attracting attention as clean and sustainable technologies. However, between these two technologies, photochemical reduction, termed artificial photosynthesis, is more advantageous as it does not require complex device design or detailed components and it is therefore less expensive to implement. Consequently, photochemical methods are gaining more interest as an environmentally sustainable means of CRR. However, high conversion efficiencies from sunlight are yet to be achieved for CRR by artificial photosynthesis, [1] for instance, through the fabrication of highly efficient photocatalysts/cocatalysts capable of high energy conversion efficiency from sunlight.

Cocatalysts for photocatalytic CRR
In CRR, semiconductor photocatalysts absorb light energy and generate excited electrons (e À ) with reduction power and excited holes (h + ) with oxidizing power (Figure 2A). However, when these semiconductor photocatalysts are used for CRR, a high conversion efficiency cannot be obtained in most cases. This is largely due to: 1) deactivation of the carriers by exciton recombination and thermal relaxation in a relatively short period of time and 2) the small number of active sites on the photocatalyst surface that can serve as reaction sites. Thus, a cocatalyst (active site) consisting of metal and/or metal oxide particles is generally loaded on the photocatalyst (Figure 2A). By loading such cocatalysts, 1) the excited e À or h + generated within the photocatalyst are transferred to the cocatalyst, thereby extending the charge separation lifetime and 2) surface atoms and electronic states are created that are suitable for adsorption and desorption of the substrate, thereby improving the reactivity and selectivity of CRR. In addition, loading of the cocatalyst is effective in preventing self-deactivation of the photocatalyst caused by its reducing or oxidizing power, thereby improving the durability of the photocatalyst.
Currently, the CRR process has the following limitations: 1) the reaction is difficult to proceed; 2) side reactions and reverse reactions are likely to occur; and 3) the selectivity of the reduction products is low. Limitation 1 is related to the fact that CO 2 has a large negative standard Gibbs energy of evolution (À 394.4 kJ mol À 1 ) and is therefore a very stable molecule. As CO 2 is more energetically stable than methanol (CH 3 OH; À 166 kJ mol À 1 ), methane (CH 4 ; À 51 kJ mol À 1 ), or ethylene (C 2 H 4 ; 68 kJ mol À 1 ), the CRR process is an uphill reaction and reduction is thus difficult to proceed. Limitation 2 is related to the fact  that the reduction potentials of CO 2 and H 2 O are comparable to each other. This causes a competitive H 2 evolution reaction (HER) to occur, in which H 2 O is reduced to H 2 , in parallel with CRR, making it difficult to only obtain the desired CO 2 -reduced species with a high selectivity ( Figure 2B). In addition, as oxygen (O 2 ) and other substances produced in O 2 evolution reaction (OER) are present in the system, a reverse reaction to CO 2 can also occur upon reaction of these products and substances. Limitation 3 is related to the fact that there are various reducing species of CRR (e. g., CO, formic acid (HCOOH), formaldehyde (HCHO), acetic acid, CH 4 , and C 2 H 4 ). Once such mixtures are obtained, their separation requires a great amount of energy. To overcome these challenges and realize artificial photosynthesis with high efficiency, high selectivity, and high durability, it is essential to improve the cocatalyst. [2] 1.3. Purpose and contents of this review As described above, the improvement of cocatalysts plays an important role in the engineering of highly functional photocatalysts. Therefore, we have conducted extensive research on improving the cocatalysts on photocatalysts. [3] Our work as well as the research conducted by other groups have provided much insights into the selection/fabrication of appropriate cocatalysts for water-splitting photocatalysts. [4] In contrast, the development of suitable cocatalysts for CRR is still in its infancy.
To realize CRR with high efficiency, high selectivity, and high durability, a comprehensive review and summary of the findings and progress achieved to date is necessary. Therefore, in the present review, we summarize representative studies conducted to date on the design and engineering of cocatalysts in CRR using semiconductor photocatalysts.
In Section 2, the mechanism of CRR with semiconductor photocatalysts is described in detail, followed by a discussion on the characteristics and types of semiconductor photocatalysts used in CRR in Section 3. In Section 4, we describe examples of previous studies and findings on controlling the properties of cocatalysts, which is the subject of this review. Section 5 provides a summary of insights gained to date and an outlook on future prospects in this area.
The focus of the present review is on the control of cocatalyst particles in CRR using semiconductor photocatalysts. Therefore, readers interested in improving CRR photocatalysts using surface plasmon resonance, [5] single-atom metal loading, [6] functional carbon, [7] or metal complex loading [8] are referred to those review articles.

Mechanism
Photocatalytic CRR generally proceeds in H 2 O or in a vapor atmosphere. CRR proceeds according to the following three major steps ( Figure 2A): [2,9] Step 1) the semiconductor photocatalyst absorbs light, which leads to the evolution of excited e À in the conduction band (CB) and excited h + in the valence band (VB); Step 2) the excited e À and h + diffuse to the photocatalyst surface and, when a cocatalyst is present, further migrate to the cocatalyst; and Step 3) CRR [Eqs. (1)-(5)] or OER [Eq. (6)] proceeds on the cocatalyst or on the photocatalyst surface.
In a typical CRR, a proton (H + ) is included in the reaction system [Eqs. (1)-(5)]. This is because of the high stability of CO 2 that requires a large reducing power of À 1.9 eV (vs. normal H 2 electrode (NHE); at pH 7) for its conversion into an anion radical species. Such a large reduction power is difficult to generate with visible-light-driven semiconductor photocatalysts. In con- trast, in multi-electron reactions using H + [Eqs. (1)-(5)], HCOOH, CO, HCHO, CH 3 OH, and CH 4 are the reaction products. The reduction potential required for these reactions to proceed is much lower than that in one-electron reactions (reactions via anion radical species). Therefore, most CRR processes that employ semiconductor photocatalysts are designed as multielectron reaction systems using H + .

Reaction system
Theoretically, CRR proceeds when the CB minimum edge (CBM) of the semiconductor photocatalyst is more negative than the reduction potential of CO 2 . OER proceeds when the VB maximum edge (VBM) of the semiconductor photocatalyst is more positive than the oxidation potential of H 2 O ( Figure 2B). Control of the CBM and VBM positions is relatively easy to achieve for semiconductor photocatalysts of sufficiently large band gaps (BGs; UV-light-driven semiconductor photocatalysts). However, as shown in Figure 3, visible light (~400 � λ �8 00 nm) accounts for about 43 % of sunlight. Therefore, it is indispensable to make effective use of sunlight using photocatalysts with small BGs, which can absorb visible light (namely, visible-light-driven semiconductor photocatalysts), for engineering semiconductor photocatalysts with high efficiencies. However, it is difficult to achieve appropriate CBM and VBM positions simultaneously in such semiconductor photocatalysts. Therefore, only a limited number of one-step photocatalytic materials ( Figure 4A) that can reduce CO 2 under visible light have been reported.
In contrast, CRR can also proceed according to a two-step reaction mechanism involving a semiconductor photocatalyst capable of instigating both CRR and OER. These reactions function by combining a photocatalyst, which causes CRR and OER, and a redox couple (mediator), which is responsible for the transfer of excitons (e À and h + ; Figure 4B). Such a system that mimics plant photosynthesis is called a Z-scheme type. When using such Z-scheme, there are significantly more types of photocatalysts available for study, and it is easier to use longer wavelength light with these photocatalysts than with the one-step photocatalysts. However, there are some limitations such as the theoretically low conversion efficiency, blocking of light absorption, and photocatalyst inactivation due to side reactions resulting from the use of a mediator.

metal elements
Typical examples of metal sulfide semiconductor photocatalysts include CdS and ZnS. [12,22] In contrast to the VB of metal oxides that is composed of O 2p orbitals, the VB of metal sulfides is composed of S 3p orbitals, which are located on the more negative side than the O 2p orbitals. Therefore, the BG of metal sulfides has a suitable width to absorb visible light, and CRR can be performed on metal sulfide photocatalysts under visible light irradiation. However, metal sulfides are readily oxidized by the h + generated upon light irradiation, resulting in self-decomposition and reduced durability of the photocatalyst. However, when the reaction is carried out in an aqueous solution containing sulfur species, self-decomposition is suppressed, and a relatively high activity is achieved. g-C 3 N 4 is an organic semiconductor photocatalyst that does not include a metal element. [13,15h,23] g-C 3 N 4 can be synthesized by a simple method of thermal polymerization [15h,24] using Ncontaining precursors (e. g., urea, melamine, and cyanamide). Moreover, these precursors are naturally abundant hence g-C 3 N 4 can be synthesized at a low cost. g-C 3 N 4 has a BG of 2.7-2.9 eV, which is suitable for visible light absorption, and the band structure is also suitable for CRR because the CBM is composed of C p z orbitals and the VBM is composed of N p z orbitals. Because of these desirable properties, g-C 3 N 4 has attracted great attention as a visible-light-driven photocatalytic material for CRR.
MOFs, which are composed of metal ions/clusters and organic linker molecules, have also been extensively studied as a photocatalytic material for CRR. [8a,14a,b,25] In MOFs, charge separation occurs efficiently because of their large specific surface area and high density of catalytic activity sites. These characteristics make MOFs attractive as a CRR photocatalyst.

Particle size control
The size of the metal NPs that serve as cocatalysts has a significant effect on CRR activity and selectivity. [38] Biswas and colleagues investigated the effect of PtNP cocatalysts of different particle sizes on CRR activity in 2012. [38a] PtNPs of different particle sizes were formed on TiO 2 (PtNPs/TiO 2 ) by changing the deposition time of Pt atoms using a sputtering method ( Figure 7A). When Pt was deposited for 20 s, small PtNPs (1.04 � 0.08 nm) were loaded on the photocatalyst. The resulting photocatalyst showed high activity (1361 μmol g À 1 h À 1 ), as assessed by the rate of CH 4 evolution, with an estimated quantum yield of 2.41 % ( Figure 7B). In comparison, when Pt was deposited for either a shorter time (e. g., 10 s) or a longer time (e. g., � 30 s), smaller (0.63 � 0.06 nm) or larger (� 1.34 � 0.15 nm) PtNPs were loaded. These resulting photocatalysts showed lower activity for CH 4 evolution than the photocatalyst loaded with 1.04 � 0.08 nm PtNPs ( Figure 7B). The different CH 4 evolution rates displayed by the PtNPs/TiO 2 photocatalysts were attributed to changes in the Fermi level of the cocatalyst ( Figure 7C). If the PtNPs are too small, electron transfer from the CB of TiO 2 to PtNPs is unlikely to occur because the PtNPs have discrete energy bands. In contrast, if the PtNPs are too large, they can easily become the recombination center for e À and h + owing to their electronic structure, which is comparable to that of the bulk metal. These results show that to improve the catalytic activity of CRR, it is essential to obtain an appropriate electronic structure, which can be achieved by controlling the size of the cocatalyst particles. Likewise, Zhang and colleagues have reported the importance of controlling the particle size of cocatalysts in improving the catalytic activity of CRR in 2018. [38b] They synthesized PtNPs on porous TiO 2 -SiO 2 (HTSO) by a simple acid-base-mediated alcohol reduction method ( Figure 8A). They investigated the relation between the size of the PtNPs and CRR activity on the obtained photocatalysts, and revealed that the photocatalysts loaded with smaller PtNPs (i. e., 1.8 nm) showed higher CH 4 evolution rate and HER rate than those loaded with larger PtNPs (3.4-7.0 nm; Figure 8B). To rationalize the results obtained, femtosecond transient absorption (fs-TA) spectroscopy and transient photocurrent response measurements were performed. The results revealed that decreasing the particle size enhanced charge transfer at the metal-support interface. Specifically, the authors suggested that the Fermi level shifted  (d). B) CO and CH 4 yields obtained during CRR on commercially available TiO 2 powder (P25), pristine TiO 2 columnar film (TiO 2 film), and PtNPs/TiO 2 films prepared by using different Pt deposition times (10, 20, 30, 45, and 60 s). C) Schematic diagram of CO 2 -photoreduction mechanism with PtNPs/TiO 2 nanostructured films. The magnified circle (center) shows that the photogenerated e À can move rapidly within the highly oriented TiO 2 single crystals and flow to the Pt deposits, where a redox reaction occurs to convert CO 2 into CO and CH 4 . The energy levels of the PtNPs/TiO 2 -CO 2 system is also shown. Reproduced with permission from ref. [38a]. Copyright: 2012, American Chemical Society. to the positive side as the particle size became smaller, which facilitated charge transfer at the interface. In contrast, larger PtNPs showed the high selectivity of CH 4 evolution. When the size of PtNPs was larger, the ratio of terrace sites, such as Pt(111) planes, increased ( Figure 8C). Density functional theory (DFT) calculations indicated that the rate-determining step of *CO to *COH hydrogenation was more likely to occur on such Pt(111) surfaces ( Figure 8D), thus explaining the higher CH 4 selectivity displayed by the larger PtNPs. These results indicate that changes in the exposed crystal faces associated with size changes of the cocatalyst also have a significant effect on the selectivity of CRR. The study demonstrates that 1) the smaller particle size of the PtNPs improves the catalytic activity of CRR and HER but 2) the generation of terrace sites is required on the surface of the cocatalyst for improving the selectivity of CH 4 evolution.

Chemical composition control
Alloying of the cocatalyst improves the adsorption of intermediates and facilitates the progression of multi-electron reactions, resulting in the improvement of the catalytic activity and selectivity in CRR. [38c,39] Xiong and colleagues successfully obtained alloy cocatalysts with isolated Cu atoms within PdNPs on TiO 2 in 2017. [39a] Furthermore, the authors found that the resulting PdCu alloy NPs-loaded photocatalysts (Pd x Cu 1 NPs/TiO 2 ; x = 1, 3, 5, 7, 9, or 11) exhibited higher CH 4 selectivity than PdNPs/TiO 2 photocatalyst in which pure PdNPs were loaded as cocatalysts. The Pd x Cu 1 NPs/TiO 2 (x = 1, 3, 5, 7, 9, or 11) photocatalysts with different alloy ratios were obtained by controlling the concentrations of the Pd and Cu salt precursors. Transmission electron microscopy (TEM) images (Figure 9Aa, b) and high-angle annular dark-field scanning TEM (HAADF-STEM) images (Figure 9Ac,d) showed that Pd x Cu 1 alloy cocatalysts were loaded on TiO 2 with similar shape and size (average particle size: 6 nm) regardless of the Cu content. Among the Pd x Cu 1 NPs/ TiO 2 photocatalysts examined, Pd 7 Cu 1 NPs/TiO 2 displayed the highest CH 4 evolution rate (19.6 μmol g À 1 h À 1 ) and selectivity (95.9 %; Figure 9B). Diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) showed that the evolution of intermediates during CRR was enhanced on the Pd 7 Cu 1 NPs/TiO 2 surface relative to the Pd 1 Cu 1 NPs/TiO 2 surface. First-principles simulations also showed that CO 2 was more strongly adsorbed on the cocatalyst surface of the PdÀ Cu pair when Cu atoms were surrounded by a higher number of Pd atoms ( Figure 9C). These two factors were interpreted as the origin of the increase in the CH 4 evolution rate and selectivity of the final product when the Cu atoms were isolated in the Pd lattice to form an alloy cocatalyst.
Bai and colleagues also reported a study on PdCu alloy cocatalysts with similar metal species in 2018. [39b] The authors investigated the CRR activity of PdCu-ordered alloy cocatalysts ( Figure 10A) formed on g-C 3 N 4 nanosheets. The results demonstrated that the Pd 1 Cu 2 NPs/g-C 3 N 4 photocatalyst annealed at 375°C in H 2 atmosphere 1) had ordered alloy layers with a body-centered cubic (bcc) structure ( Figure 10A) and 2) had a higher CH 4 evolution rate (4.95 μmol h À 1 g À 1 ) and selectivity (96.5 %) than the other examined PdCuNPs/g-C 3 N 4 photocatalysts with different composition ratios ( Figure 10B). The authors further found that in the PdCu-ordered alloy cocatalyst, 1) the electronic interaction between Pd and Cu was enhanced, thus increasing their electron trapping capacity and 2) isolated Cu sites were exposed on the cocatalyst surface ( Figure 10A). The origin of the high catalytic activity and selectivity of the Pd 1 Cu 2 NPs/g-C 3 N 4 photocatalyst was attributed to these two phenomena.
In a study by Zhao and colleagues in 2022, the authors investigated the selectivity and mechanism of CRR using photocatalysts loaded with PtCu alloy NPs composed of Pt and Cu. [39c] PtCuNPs/TiO 2 were prepared by the H 2 reduction method. X-ray photoelectron spectroscopy measurements, STEM measurements, and energy-dispersive X-ray spectroscopy measurements confirmed the loading of the alloy NPs, consist-ing of Pt and Cu, on TiO 2 . The results of the catalytic activity study revealed that Pt 0.4 Cu 0.6 NPs/TiO 2 did not produce H 2 , and CH 4 was obtained at 100 % selectivity ( Figure 11A). Multiple experiments showed that alloying 1) suppressed the recombination of the photogenerated carriers and 2) enhanced charge transfer from the photocatalyst to the cocatalyst. In-situ Fourier transform infrared spectroscopy (FTIR) and DFT calculations were performed to elucidate the mechanism of the selective evolution of CH 4 . The results suggested that the selective reduction to CH 4 occurred by the following mechanism: 1) CO 2 molecules adsorb chemically on the surface of the catalyst and they trap e À , producing the intermediate CO 2 À ( Figure 11B); 2) CO 2 À causes the evolution of *CO, an important intermediate species for the generation of CH 4 and 3) activation and hydrogenation of *CO is promoted ( Figure 11C). The authors suggested that in this mechanism, desorption of *CO from the PtCu alloy catalyst surface is difficult because *CO and PtCuNPs form a strong bond ( Figure 11D), resulting in increased CH 4 selectivity and reaction activity.

Morphological control
The shape and crystal facets of the metal cocatalyst have significant effects on adsorption of CO 2 and thereby the catalytic activity. Therefore, it is important to form cocatalysts with appropriate facets to obtain the desired outcome. Such optimum shape and crystal facets depend on the metal species of the cocatalyst. [39d,40] In 2014, Xiong and colleagues examined two types of photocatalysts loaded with differently shaped PdNP (~4-6 nm) cocatalysts (cubic; cube-PdNPs and tetrahedral; tetra-PdNPs; Figure 12A) to investigate the effect of metal cocatalyst shape on CRR activity. [40a] The PdNPs cocatalysts of different shapes were formed on g-C 3 N 4 using appropriate capping agents in the liquid phase. As confirmed from the high-resolution (HR) TEM images in Figure 12B, the NPs in the form of cube-PdNPs and tetra-PdNPs were loaded on the photocatalyst (cube-PdNPs/g-C 3 N 4 and tetra-PdNPs/g-C 3 N 4 ) and were composed of Pd(100) and Pd(111) planes, respectively. Measurements of the CRR activity revealed that tetra-PdNPs/g-C 3 N 4 displayed approximately four times higher carbon product (CH 4 , C 2 H 5 OH, CO) selectivity than cube-PdNPs/g-C 3 N 4 . The photocurrent and photoluminescence measurements indicated that differences in the cocatalyst shape had no effect on charge transfer from g-C 3 N 4 to PdNPs. Therefore, simulation calculations were performed, which showed that the Pd(111) facet displayed a higher CO 2 adsorption energy (E a ; Figure 12C) and a lower activation energy barrier than the Pd(100) facet. These results suggest that loading of tetra-PdNPs composed of Pd(111) planes is effective in engineering appropriate photocatalysts for CRR using Pd as a cocatalyst element.
The effect of cocatalyst shape on selectivity during CRR was also reported by Yu and colleagues in 2017. [40b] PdNPs of different shapes were synthesized using different capping agents and subsequently loaded onto g-C 3 N 4 by electrostatic interaction. From the CRR activity measurements, it was found

Chemistry-A European Journal
Review doi.org/10.1002/chem.202203387 that tetra-PdNPs/g-C 3 N 4 displayed 1.42 times higher CH 3 OH evolution rate than cube-PdNPs/g-C 3 N 4 ( Figure 13A). In situ FTIR spectrum of tetra-PdNPs/g-C 3 N 4 during photoirradiation displayed absorption bands related to the evolution of intermediates (HCOOH and HCHO; Figure 13B). This result suggests that CRR on tetra-PdNPs/g-C 3 N 4 surfaces proceeds according to a multistep mechanism involving the evolution of intermediates, such as HCOOH and HCHO, which are subsequently converted into CH 4 and CH 3 OH. In addition, DFT calculations showed that 1) CO 2 was more strongly adsorbed on tetra-PdNPs than on cube-PdNPs at all sites of the bridge, Top1, and Top2 positions (Figure 13Ca-c) and 2) the product, CH 3 OH, more readily desorbed from the Pd(111) surface than from the Pd(100) surface (Figure 13Cd). They concluded that tetra-PdNPs/g-C 3 N 4 with a Pd(111) facet exhibit high CRR activity due to these factors.
This shape dependence was also observed for photocatalysts featuring alloy cocatalysts. In 2017, Bai and colleagues reported that the rate and selectivity of CH 4 evolution was dependent on the crystal facet of PtCu alloy cocatalysts. [39d] PtCu alloy cocatalysts were formed on g-C 3 N 4 by hydrothermal reduction of a solution containing the metal salts (K 2 PtCl 4 , CuCl 2 ) and polyvinylpyrrolidone. The shape of the cocatalyst was controlled by adjusting the amount of hydrogen chloride (HCl), which acts as an oxidative etching agent, to obtain cube-PtCuNPs/g-C 3 N 4 ( Figure 14Aa) and concave cube-PtCuNPs/g-C 3 N 4 (with a concave surface; Figure 14Ab). The HRTEM measurements revealed that the (100) and (730) facets were exposed in each cocatalyst (Figure 14Ac, d). The concave cube-PtCuNPs/g-C 3 N 4 displayed CO and CH 4 evolution rates of 0.046 and 0.112 μmol h À 1 , respectively. These evolution rates were respectively about two and three times higher than the CO and CH 4 evolution rates obtained on cube-PtCuNPs/g-C 3 N 4 (Figure 14B). Furthermore, concave cube-PtCuNPs/g-C 3 N 4 exhibited a CH 4 selectivity of 90.6 %, which was higher than that (85.9 %) displayed by cube-PtCuNPs/g-C 3 N 4 . The DFT calculations suggested that the adsorption energy of CO 2 was higher on the PtCu(730) facet than on the PtCu(100) facet, indicating that the PtCu(730) facet had superior CO 2 molecular adsorption capacity ( Figure 14C). In addition, there is generally a relatively strong electronic interaction between the Pt atom in the lowcoordinated state and CO 2 . These results explained the high CH 4 evolution activity and selectivity displayed by concave cube-PtCuNPs/g-C 3 N 4 .

Surface structure control
To improve the activity and selectivity of CRR, it is also important to suppress the competing HER and reverse reaction that produces CO 2 . Therefore, several groups have attempted to form cocatalysts with a core-shell structure using two different metal species. [41] In 2013, Wang and colleagues reported the synthesis of cocatalysts with a core-shell structure composed of Pt and Cu 2 O and their high selectivity for the evolution of CO and CH 4 . [41a] A photodeposition method was used to form PtNPs on TiO 2 with a particle size of about 3.1 nm. Copper sulfate was then added to form Cu 2 O/PtNPs/TiO 2 , in which PtNPs (core) were covered by Cu 2 O (shell) in a stepwise photodeposition. The thickness of the shell layer composed of Cu 2 O was controlled by the light irradiation time (x h) (Cu 2 O/PtNPs/TiO 2x h). As confirmed by TEM analysis, the Cu shell completely covered the PtNPs after light irradiation for 5 h (Cu 2 O/PtNPs/ TiO 2 -5 h; Figure 15A). The evolution of the shell layer after 5 h of light irradiation was also confirmed by high-sensitivity lowenergy ion scattering (HS-LEIS) measurements ( Figure 15B). Cu 2 O/PtNPs/TiO 2 -5 h, with a Cu content of 3 wt%, showed a CRR selectivity of 85 % ( Figure 15C). Within this core-shell configuration, the PtNPs as the core have a high electron capture capacity, while the Cu 2 O as the shell suppresses the reduction of H 2 O to H 2 . Therefore, the core-shell configuration of this Cu 2 O/PtNPs/TiO 2 photocatalyst appears to be conducive to suppressing the competing HER and enabling CRR on the photocatalyst.
In contrast, in the core-shell-type cocatalyst reported in 2018 by Tanaka and colleagues, the reverse reaction of CRR was significantly suppressed instead of the HER ( Figure 16A). [41b] The core-shell-type cocatalysts consisting of AgNP cores and chromium hydroxide (Cr(OH) 3 ) shells were formed on Ga 2 O 3 by photodeposition (Cr(OH) 3 · xH 2 O/AgNPs/Ga 2 O 3 ). HRTEM analysis confirmed the formation of Cr(OH) 3 · xH 2 O shell of about 3-5 nm thickness on the AgNP surface ( Figure 16B). The obtained Cr(OH) 3 · xH 2 O/AgNPs/Ga 2 O 3 photocatalyst exhibited an evolution rate of 480 μmol h À 1 and a selectivity of 83.8 % for the conversion of CO 2 into CO. These values were respectively 2.4 and 2.0 times higher than those of AgNPs/Ga 2 O 3 photocatalyst without a core-shell structure. Cr(OH) 3 · xH 2 O, which forms the shell layer, is assumed to change to the carbonate compound, Cr 2 (OH) 2m (CO 3 ) (3-m) · xH 2 O, in NaHCO 3 solution. This shell layer of carbonate compounds is believed to provide continuous supply of CO 2 molecules to the AgNP core, while preventing the approach of O 2 , thereby suppressing the reverse reaction on
In 2019, Tanaka and colleagues studied the effect of shell thickness on the photocatalytic activity and selectivity for the same photocatalyst. [41c] The studies revealed that 1) high CO evolution rates were obtained when Cr(OH) 3 · xH 2 O/AgNPs/ Ga 2 O 3 was prepared at Ag : Cr ratios of 1 : 1 and 2) the highest CO evolution rate (525.3 μmol h À 1 ) and selectivity (85.2 %) were obtained for photocatalysts loaded with Ag and Cr both at 0.25 wt%. Examination of the chemical composition and shape of the Cr shell on the photocatalyst revealed that Cr(OH) 3 · xH 2 O was transformed to Cr(OH) x (CO 3 ) y during the photocatalytic reaction ( Figure 17A). Furthermore, the rate of CO evolution was significantly dependent on the thickness of the shell layer ( Figure 17B). When a shell layer with appropriate thickness was formed on the surface of the Ag core cocatalyst, CO 2 was stably trapped within the active site, allowing CO 2 to be preferentially reduced to CO ( Figure 17C). However, a thicker shell layer would prevent the approach of carbon species and protons to the Ag core. These results reveal that the formation of a shell layer of appropriate thickness is key to achieve high catalytic activity and selectivity. Such functional shells have been reported to also form using carbon [7b,42] and NiO. [41d,v]

Summary and Outlook
The area of CRR with semiconductor photocatalysts is still in its developing stage, with knowledge acquired from extensive studies conducted in the area. To contribute to future advances in engineering CRR photocatalysts with high efficiency, high selectivity, and high durability, this review has provided a summary of the major findings of recent studies conducted in the field, with a focus on the effects of controlling the properties of metal NP cocatalysts on CRR efficiency. The main insights and discussion points are as follows: 1) A decrease in the size of metal NP cocatalysts affords an increase in the number of active sites owing to an increase in the specific surface area. However, if the size is excessively reduced, the electronic states become discretized. Therefore, electron transfer from the photocatalyst to the cocatalyst is disrupted, potentially leading to a reduction in activity. In addition, changes in particle size lead to changes in the exposed crystal facet. Therefore, to achieve high selectivity and activity, the loading of metal NPs of suitable size is important. 2) Alloying PdNPs or PtNPs with Cu suppresses recombination of the photogenerated carriers, enhances charge transfer from the photocatalyst to the cocatalyst, and stabilizes the reaction intermediates, enhancing the selectivity and rate of CH 4 evolution. If isolated Cu sites and ordered alloying are achieved, the selectivity of the products can be further enhanced. 3) Control of the exposed crystal facets of the metal NP cocatalysts contributes to the improvement of the activity and selectivity of the CRR. In the PdNP cocatalyst, metal NPs with exposed Pd(111) surfaces are suitable cocatalysts for CRR. In the PtCu alloy cocatalyst, exposure of the PtCu(730) surface leads to highly efficient and selective CH 4 evolution. 4) Suppression of the competing HER or the reverse reaction improves the activity and selectivity of CRR. In particular, the formation of shell layers can be effectively used to suppress these reactions. Given the knowledge gained to date, further areas remain to be explored, as summarized below, to guide the development of future CRR photocatalysts. 1) CRR is a multi-electron reaction that employs several electrons simultaneously, and a longer charge separation lifetime contributes significantly to the activity. It is expected that the carrier lifetimes of various photocatalysts will be elucidated by fluorescence lifetime measurements, TA spectroscopy, [43] and time-resolved microwave spectroscopy, [44] and that photocatalysts and cocatalysts with high charge separation efficiency will be designed and developed based on the knowledge obtained. [26ba,45] 2) Previous studies have demonstrated that the catalytic activity can be enhanced when cocatalysts are loaded on suitable crystal facets for the reduction and oxidation reactions. [28c,30n,o,46] Therefore, it is expected that CRR photocatalysts with improved activity and selectivity will be developed in the future by establishing methods to selectively load cocatalysts that have size, alloy structures,

Chemistry-A European Journal
Review doi.org/10.1002/chem.202203387 crystal facets, and shell structures suitable for achieving high activity and selectivity on the crystal facets of the photocatalysts where CRR occurs. 3) It is expected that the geometric structure of highly active and selective cocatalysts will be revealed at the atomic level by aberration-corrected TEM and STEM [47] measurements, thereby providing a clearer understanding of the effect of surface structure on catalytic activity. 4) Atomically precise metal nanoclusters (NCs), [48] whose geometric/electronic structures have been revealed by singlecrystal X-ray structural analysis and DFT calculations have been reported to be highly active cocatalysts for photocatalytic water-splitting reaction and other catalytic reactions. In addition, the use of atomically precise metal NCs as cocatalysts is also beneficial to obtain deeper understanding of the reaction mechanisms. Indeed, we have succeeded in revealing the reason why Pd doping of AuNC cocatalysts decreases the water-splitting activity, whereas Pt doping of AuNC cocatalysts increases it by using the atomically precise metal NCs as a cocatalyst. [3d] Moreover, single-atom (SA) catalysts have been reported to be efficient cocatalysts for photocatalytic CRR, [49] and their use also seems to be beneficial for obtaining deeper understanding of the reaction mechanisms. In future studies, it is expected that deeper understanding will be obtained of the reaction mechanisms in CRR by using the atomically precise metal NCs and SAs as cocatalysts and through performing operando measurements [50] such as in-situ X-ray absorption fine structure, in-situ hard X-ray photoelectron spectroscopy, and DRIFTS. 5) DFT calculations are important for understanding the ratelimiting step of a reaction. [51] However, at present, metal NP cocatalyst structures are often calculated in a simplified manner owing to computational cost issues. In the future, theoretical calculations on actual cocatalysts are expected to be conducted, providing deeper understanding of the reaction mechanisms and thereby providing clear design guidelines for the development of highly active and selective catalysts. 6) It is expected that researchers in the field of photocatalysis, as well as those in the fields of metal NC chemistry, [52] surface spectroscopic chemistry, [53] and theoretical chemistry [54] will actively engage in the development of materials for application in the CRR, thereby significantly advancing the field of CRR photocatalysts. 7) Although the number of published papers on the photocatalytic conversion of CO 2 increases year by year, the current state of the art is uneven. Namely, of the previous Figure 13. A) Evolution yields of CH 4 and CH 3 OH during CRR on different photocatalysts -CN: g-C 3 N 4 only, CN-1: photocatalyst subjected to the same treatment but without PdNPs, cube: cube-PdNPs/g-C 3 N 4 , and tetra: tetra-PdNPs/g-C 3 N 4 . B) In-situ FTIR spectra of tetra-PdNPs/g-C 3 N 4 subjected to different CRR conditions. C) Optimized geometrical structures of CO 2 adsorption on Pd(100) facets or Pd (111)

Chemistry-A European Journal
Review doi.org /10.1002/chem.202203387 studies, some do not satisfy the requirements of the field of photocatalytic conversion of CO 2 . The important requirements are as follows: 1) isotope experiments should show the carbon source of the reduction product to be CO 2 (not contamination); 2) the competing H 2 production must also be analyzed accurately, and the number of excited electrons consumed in the production of the CO 2 reduction products must be greater than those in H 2 production; 3) the oxidation product (in most cases, O 2 ; the oxidation product of H 2 O) must be analyzed accurately, and the ratio of excited electrons to holes consumed in the reaction must be 1. In future studies, these requirements are expected to be inevitably satisfied to avoid misunderstanding and thereby establish clear design guidelines for high-performance CRR photocatalysts.

Conflict of Interest
There are no conflicts to declare.

Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.

Figure 17.
A) Cr K-edge X-ray absorption near-edge structure spectra of Cr(OH) 3 · xH 2 O (black), Cr(OH) x (CO 3 ) y (red), as-prepared 1.0 mol% Cr-(OH) 3 · xH 2 O/AgNPs/Ga 2 O 3 (blue), and Cr(OH) 3 · xH 2 O/AgNPs/Ga 2 O 3 after photoirradiation for 5 h (pink). B) Dependence of the evolution rates of CO ( * ) and H 2 (~) on the thickness of the Cr(OH) 3 · xH 2 O shell with various loading amounts of Ag and Cr. Dotted lines represent the data-fitted curves. C) Schematic illustration of the mechanism of the photocatalytic conversion of CO 2 into CO on Cr/Ag/Ga 2 O 3 . Reproduced with permission from ref.